1. Field of the Invention
This invention relates to the sealing of a sodium-containing region in an electrochemical device, such as a cell utilising liquid sodium and solid electrolyte permeable to sodium ions. The invention has particular application to electrochemical cells, such as for example sodium sulphur cells, making use of sodium as one of the electrode materials.
2. Prior Art
A sodium sulphur cell contains liquid sodium separated by a sodium-ion permeable solid electrolyte, usually beta-alumina, from a cathodic reactant comprising liquid sulphur and sodium polysulphides. The sodium and the cathodic reactant are highly reactive materials and it is essential that the cell should be properly sealed to prevent any escape of these materials. Various types of seals have been proposed heretofore. Mechanical hermetic seals are disclosed for example in U.S. Pat. Nos. 3,946,751 and 3,959,013.
The present invention is concerned more particularly with glass seals. The use of glass seals is described, for example in U.S. Pat. Nos. 3,928,071, 3,826,685 and 3,868,273. It is convenient, in sodium sulphur cells and similar electrochemical cells to use a glass as a bonding agent between a ceramic material and a metal member. In a sodium sulphur cell, the closure may be effected by sealing the ceramic electrolyte material to a closure member or to the housing. It is well-known however to put an alpha-alumina extension onto a beta-alumina ceramic tube to have a non-conductive end portion of the tube. This may readily be done with a glass seal. The alpha-alumina extension then has to be sealed to the housing or to the closure member. The closure member may be a part of a current collector. Thus glass may be employed, in sealing a cell, as a bond between solid electrolyte material, e.g. beta-alumina ceramic, or an insulating ceramic, e.g. alpha-alumina, and a metal component or components such as a current collector, an intermediate component, or an external housing. The glass-to-metal bond is formed by a reaction between the glass and an oxide layer on the metal. The glass employed is a sodium-resistant glass, such as an aluminate or alumino-borate glass. The metal material has to be chosen in accordance with both mechanical and chemical requirements. In particular, it must resist attack by sodium at elevated temperatures. It is the practice in sodium sulphur cells to use mild steel or stainless steel for the housing, in contact with the sodium.
The glass-to-steel bond formed is an oxide bond and applicant has found that the oxide material of the bond is reduced leading to bond weakening and eventually seal failure, when this bond is exposed to sodium vapour. The problem arises very particularly at the elevated temperatures employed in electrochemical cells using liquid sodium which may typically operate at temperatures of 350.degree. to 400.degree. C. Although techniques have been devised for sealing metal direct to ceramic material so as to avoid the use of glass, there are substantial advantages in the employment of glass as a bonding agent in electrochemical cells of the kind described above. The present invention is directed to preventing or minimising glass-to-metal bond weakening in the presence of sodium vapour.